FIG. 1 is a typical wet milling process for alcohol production. FIG. 2 is a typical dry milling process with a back-end oil recovery system. FIG. 3 is a typical dry milling process with a back-end oil and protein recovery system.
Conventional methods of producing alcohols from grains generally use two procedures. One of the procedures is operated in a wet condition and the other is operated under a dry condition, which are referred to as a wet milling process and a dry milling process respectively. The wet milling corn processing plants convert corn grains into several different co-products, such as germs (for oil extraction), gluten feed (high fiber animal feed), gluten meal (high protein animal feed), and starch-based products (such as ethanol, high fructose corn syrup, and food) and industrial starch. The dry grind ethanol plants convert corns into two products including ethanol and distiller's grains with soluble. The distiller's wet grains with soluble is referred to as DWGS if it is sold as wet animal feed. The distiller's dried grains with soluble is referred to as DDGS if is dried to be used as an animal feed.
In the typical dry grinding mill process for ethanol production, one bushel of corn yields approximately 8.2 kg (approximately 17 lbs.) of DDGS in addition to an approximately 10.3 liters (approximately 2.75 gal) of ethanol. These co-products provide a critical secondary revenue stream that offsets a portion of the overall ethanol production costs. DDGS is typically sold as a low value animal feed even though that the DDGS contains 11% oil and 33% protein. Some plant starts to modify the typical processes by separating the valuable oil and protein from the DDGS.
It is reported that there are about 40 plants using a back-end oil recovery system, one plant having a protein recovery system, and one plant having a front grind milling with a front oil recovery system. These improved processes have the same goal that is to increase an alcohol yield of the plants as well as to recover valuable oil from the front-end process. Generally, a front-end process refers to steps and/or procedures that are performed before fermenting and a back-end process refers to steps and/or procedures that are performed after the fermenting.
In the following, some typical wet milling processes are disclosed. FIG. 1 is a flow diagram of a typical wet milling ethanol production process 10. The process 10 begins with steeping 11, in which corns (corn kernels that contain mainly starch, fiber, protein, and oil) are soaked for 24 to 48 hours in a solution of water and sulfur dioxide to soften the kernels for grinding. In the steeping 11, soluble components leach into the steep water and the protein matrix and the endosperm are loosened. Next, the steeped corn (after the steeping 11) with about 50% of DS is fed to a determination milling 12 (first grinding) at a grinding mill, in which the corn is ground in a manner that tears open the kernels and releases the germ so as to make a heavy density (8 to 9.5 Be) slurry of the ground components, which is primarily a starch slurry.
Next, germ separating 13 is performed by floating germs and a hydrocyclone(s) is used to separate the germ from the rest of the slurry. The germs contain oil, which are inside the kernel. The separated germs in a stream (separated out as a germ byproduct) contain some portions of starch, protein, and fiber. The separated germs are sent to a germ washing 13A, such that the starch and protein are able to be removed. Next, the germ stream is sent to a dryer. About 2.5 to 3 lbs. (dry basis) of germs per bushel of corn are generated. The dry germs have about 50% of oil content on a dry basis.
The remaining slurry from the germ separating 13, which is now devoid of germs containing fiber, gluten (e.g., protein), and starch, is subjected to fine grinding 14 (second grinding) at a fine grinding mill, where total disruption of endosperm occurs. The endosperm components are released (including gluten and starch) from the fiber.
Next, fiber separating 15 is performed. In the fiber separating 15, the slurry passes through a series of screens to separate the fibers from the starch and gluten. The fibers are washed to be clean of the gluten and starch. The fiber separating 15 typically employs static pressure screens or rotating paddles mounted in a cylindrical screen (paddle screens). Even after washing, the fibers from a typical wet grinding mill still contain 15%˜20% of starch. This starch is able to be sold with the fibers as animal feed. The remaining slurry, which is now devoid of fiber, is subjected to gluten separating 16, in which the centrifugations separate starch from the gluten. The gluten stream (at gluten filtrating and drying 16A) goes to a vacuum filter followed by a drying step at a dryer to produce gluten (protein) meal.
Next, liquefying/saccharifying 17, fermenting 18, distilling/dehydrating 19 are performed. At the liquefying/saccharifying 17, the starch from the starch gluten separating 16 goes through a jet cooker to start the process that converts the starch to sugar. Jet cooking refers to a cooking process that is performed at elevated temperatures and pressures. The elevated temperatures and pressures are able to be varied widely. Typically, jet cooking occurs at a temperature about 120° C. to 150° C. (about 248° F. to 302° F.) and a pressure about 8.4 kg/cm2 to 10.5 kg/cm2 (about 120 lbs./in2 to 150 lbs./in2), although the temperature is able to be as low as about 104° C. to 107° C. (about 220° F. to 225° F.) when a pressure of about 8.4 kg/cm2 (about 120 lbs./in2) is used. Liquefying occurs when the mixture or “mash” is held at 90° C. to 95° C. Under such condition, alpha-amylase hydrolyzes the gelatinized starch into maltodextrins and oligosaccharides (chains of glucose sugar molecules) to produce a liquefied mash or slurry. The process of saccharifying is performed by cooling the liquefied mash to about 50° C. and adding a commercial available enzyme known as gluco-amylase. The gluco-amylase hydrolyzes the maltodextrins and short-chained oligosaccharides into single glucose sugar molecules to produce a liquefied mash.
In fermenting 18, a common strain of yeast (Saccharomyces crevasse) is added to metabolize the glucose sugars into ethanol and CO2. Upon completion, the fermented mash (“beer”) contains about 17% to 18% ethanol (volume/volume basis). Subsequent to the fermenting 18 is the distilling and dehydrating 19, in which the beer is pumped into distillation columns where it is boiled to vaporize the ethanol. The ethanol vapor is condensed in the distillation columns, and liquid alcohol (e.g., ethanol) exits the top of the distillation columns at about 95% purity (190 proof). Next, the 190 proof of ethanol goes through a molecular sieve dehydration column, which removes the remaining residual water from the ethanol, such that a final product of essentially 100% of ethanol (199.5 proof) is produced. This anhydrous ethanol is now ready to be used for motor fuel purposes. The solids and some liquid remaining after distilling go to evaporating 20, where yeast is able to be recovered as a byproduct. Yeast is able to be optionally recycled back to the fermenting 18. In some embodiments, the CO2 is recovered and sold as a commodity product. The “stillage” produced after distilling and dehydrating 19 in the wet milling process 10 is generally called “syrup.”
The wet grinding process 10 is able to produce a high quality starch product that is able to be converted to alcohol, as well as separate streams of germs, fiber and protein, which are able to be sold as byproducts to generate additional revenue streams. However, the wet grinding process is complicated and costly requiring high capital investments as well as high-energy costs for operation.
Because the capital costs of wet grinding mills are so prohibitive, some alcohol plants prefer to use a simpler dry grinding process. FIG. 2 is a flow diagram of a typical dry grinding ethanol production process 20. As a general reference point, the dry grinding ethanol process 20 is able to be divided into a front-end and a back-end process. The part of the process 20 that occurs prior to fermenting 23 is considered the “front-end” process, and the part of the process 20 that occurs after fermenting 23 is considered the “back-end” process.
The front-end process of the process 20 begins with grinding 21, in which dried whole corn kernels are passed through hammer mills to be ground into corn meal or a fine powder. The screen openings in the hammer mills are typically of a size 7, or about 2.78 mm, with the resulting particle distribution yielding a very wide spread and bell type curve particle size distribution, which includes particle sizes as small as 45 micron and as large as 2 to 3 mm. The ground meal is mixed with water to create slurry and a commercial enzyme called alpha-amylase is added (not shown). This slurry is then heated to approximately 120° C. for about 0.5 to three (3) minutes in a pressurized jet cooking process in order to gelatinize (solubilize) the starch in the ground meal. It is noted that in some processes a jet cooker is not used and a longer hold time is used instead.
The grinding 21 is followed by liquefying 22, whereat the ground meal is mixed with cook water to create slurry and a commercial enzyme called alpha-amylase is typically added. The pH is adjusted here to about 5.8 to 6 and the temperature is maintained between 50° C. to 105° C., so as to convert the insoluble starch in the slurry to become a soluble starch. The stream after the liquefying 22 has a content of about 30% of dry solids (DS) with all the components contained in the corn kernels, including sugars, protein, fiber, starch, germ, grit, and oil and salt. There are generally three types of solids (fiber, germ, and grit) with similar particle size distribution in the liquefying stream.
The liquefying 22 is followed by a simultaneous saccharifying and fermenting 23. This simultaneous process is referred to in the industry as “Simultaneous Saccharification and Fermentation” (SSF). In some commercial dry grinding ethanol processes, saccharifying and fermenting occur separately (not shown). Each of the individual saccharifying and SSF is able to take as long as about 50 to 60 hours. In the fermenting 23, sugar is converted to alcohol using a fermenter. Next, distilling and dehydrating 24 are performed, which utilizes a still to recover the alcohol.
In the back-end process of the process 20, which follows distilling and dehydrating 24, preconcentrating 28, fiber separating 25 (centrifuging the “whole stillage” produced at the distilling and dehydrating 24, such that the insoluble solids (“wet cake”) is able to be separated from the liquid (“thin stillage”)), and evaporating 27.
The “wet cake” from the distilling and dehydrating 24 includes fiber (per cap, tip cap, and fine fiber), grit, germ particle and some protein. The liquid from the centrifuge contains about 6% to 8% of DS, which contains mainly oil, germ, fine fiber, fine grit, protein, soluble solid from the fermenter and ash from corns. The whole stillage at some plant having about 12 to 14% of DS, which is fed to preconcentrating 28 of a first stage evaporator to concentrate the whole stillage to 15 to 25% of DS before feeding the whole stillage to the fiber separation step 25.
At the fiber separating 25, a decanter centrifuge is used to split the whole stillage into two streams (a cake stream and a liquid stream). The cake stream contains mainly fiber and sine protein, grit and germ particle. The liquid stream, which is commonly called a thin stillage, contains insoluble solid (such as protein, germ and fine fiber) and soluble solid from corn. Next, the thin stillage is split into two streams. One stream includes about 30%—40% of flow is recycled back (as a “back-set” stream) to be mixed with corn flour in a slurry tank at the beginning of the liquefying 22. The other stream containing the rest of the flow (about 60 to 70% of the total flow) enters evaporators in evaporating 27 to boil away moisture leaving a thick syrup that contains mainly fine solid (protein, germ and fine fiber) and soluble (dissolved) solids from the fermenting (25% to 40% dry solids).
The back-set water is used as part of cooking water in the liquefying 22 to reduce the fresh water consumption as well as save evaporating energy and equipment costs.
The concentrated slurry from the evaporating 27 is able to be subjected to back-end oil recovering 26, where the slurry is able to be centrifuged to separate oil from the syrup. The oil recovered is able to be sold as a separate high value product. The oil yield is normally about 0.4 lbs./Bu of corn with a high free fatty acid content. This oil yield only accounts for about ¼ of the oil in the corn. About one-half of the oil of the corn kernel remains inside the germ after the distilling 24, which cannot be separated in a typical dry grind process that uses centrifuges. The free fatty acids, which are created when the oil is held in the fermenter for approximately 50 hours, reduce the value of the oil.
The (de-oil) centrifuges is able to remove only less than 50% oil in the syrup because the protein and oil make an emulsion, which cannot be satisfactorily separated. Although adding chemicals, such as emulsion breaker, is able to improve the separation efficiency in some degrees, the chemicals are costly and the DDGS product is able to be contaminated by the added chemicals. In some cases, heat is provided or the feed temperature is raised at the centrifuge to break the emulsion, but the method affects the color and quality of DDGS. In some other cases, alcohol is added to break the emulsion, which is also able to improve the separation and increases the oil yield. However, alcohol adding needs exploration proof equipment's and costly operations. All those improvements only increase the oil yield from an average of 0.4 lbs./Bu to about average 0.6 lbs./Bu even though the “free” oil (extractable oil) in the whole stillage is about 1 lbs./Bu. The main reason for having such a low oil yield in the back-end of the typical method is that the oil and protein form emulsion during the whole dry mill process, which makes the oil recovery difficult.
An oil and protein recovery process is developed by oil/protein separating that is added to break this oil/protein emulsion of a whole stillage. As shown in the process 30 of FIG. 3, the front-end process is similar to the typical dry mill process. The process changes its procedures after the fiber separating 25 in the back-end process. This oil/protein separating 31 is able to be added between the fiber separating 25 and evaporating 27. The nozzle centrifuges, other types of disc centrifuges, or decanters are normally used for this case.
The thin stillage from the fiber separating 25 is fed to oil/protein separating 31. The oil/protein emulsion is broken up in a higher G force inside the centrifuge. The oil is in a light phase (overflow) discharge and protein is in a heavy phase discharge (underflow), because of the density difference between oil (density 0.9 gram/ml) and protein (1.2 gram/ml).
The light phase (overflow) of the oil/protein separating 31 is fed to evaporating 27 to be concentrated to contain 25%˜40% of DS (forming a semi-concentrated syrup). Next, the semi-concentrated syrup is sent to back-end oil recovering 26 to recover oil in the back-end process. The light phase stream contains less protein, so it has less chance to form oil/protein emulsion. The oil yield with this system is able to be as high as 1 lb./Bu. The de-oil syrup from the back-end oil recovering 26 is able to be further concentrated in an evaporator to a much higher syrup concentration as high as 60% of DS. The de-oil syrup with low protein is able to avoid fouling at the evaporator.
The underflow from oil/protein separating 31 is sent to a protein dewatering 32, such that the protein is able to be recovered. The separated protein cake from the protein dewatering 32, with a content having less than 3% of oil, is sent to protein drying 33 at a protein dryer to produce high value protein meal, which has a 50% of protein. The liquid from the protein dewatering 32 is sent back to the front-end as a back-set stream that is used as part of cooking water in the liquefying 22.
All of the oil that is recovered from the back-end oil recovering system has poor quality, dark color, and high fatty acid around (15 to 20%), because the oil is in the fermenter more than 50 hours. The back-end oil separation is also able to be difficult to be carried out, because the oil and protein form emulsion during the whole dry milling process. Each step in the whole dry milling process, such as pump and separation create some oil/protein emulsion. In order to get good quality oil and avoid the formation of the oil/protein emulsion during whole dry milling process, recovering oil in the front-end is able to be a good solution.
The three phases decanter that are used to recover the oil from the liquefied starch stream at the liquefying are tested, but because the high viscosity in the liquefied starch solution plus most oil still in a germ form, the oil yield is normally low at around 0.2 lbs./Bu. Nonetheless, the oil quality is much better than oil obtained from the back end having a much lighter color with about 5 to 9% of free fatty acid.